1. Field of the Invention
The present invention relates to a synchronous motor device for timepiece, and in particular to a synchronous motor device for timepiece which converts an alternating electric signal from a time signal generating means to mechanical movement of constant speed rotation.
2. Description of the Prior Art
In order to convert time generating signals of pulse waves, etc. supplied by an AC commerical power source, crystal oscillator, or the other oscillating means with a highly accurate frequency to mechanical rotation of time indicating hands, various synchronous motors for timpieces find wide utility in analog type display timepieces with accuracy. For this type of synchronous motor device small electric power consumption and reliable self-starting properties are demanded. The conventional synchronous motor, however, could not completely meet these demands.
In the prior art devices many kinds of attempts have been made to improve the properties of power consumption and self-starting rotation, wherein, in ordinary cases, the requested properties have been improved by modification of the shapes of stator poles which receive the alternating electric signal.
In the prior art devices with improved synchronous motors for timepiece, formations of a plurality of magnetized stator poles having different air gaps between stator poles and a rotor separately determines the static magnetic center of the stator (static stable position of the rotor) and the dynamic magnetic center (dynamic stable position of the rotor), which enables preferable self-starting rotation with less power consumption.
Such prior art devices mentioned above can provide preferable properties which cannot be obtained from a stator always having a fixed air gap.
In the prior art devices of such kind, however, there arises such a problem that the driving force for rotation obtained from the stator in accordance with rotating position of the rotor changes one time after another in complexity, and the consequent rotor rotation with pulsation causes inability to obtain smooth rotation and particularly smooth self-starting rotation.
In order to prevent the rotor from rotation with pulsation, one mechanically absorbs the pulsation by a damper connected to the rotor shaft, but this means causes a small increase of mechanical loss in operation and low productivity in assembling process.
An electric improvement can be offered to delete such pulsation in rotation of the rotor on the other hand. For example, according to Japanese Pat. No. 56-53274, the pulsating components are decreased and the rotor rotation is stabilized by such means that the alternating electric signal shown by A in FIG. 1 is converted to an electric signal of complicated shapes shown by B in FIG. 1 and a control pulse 500 is provided to the rotor at the final stage of the respective drive pulses.
In this prior art device, however, there arises such a problem that the circuits must be very complex in order to form the pulse signal as is shown by B in FIG. 1.